At 60% health, he transitions to Phase 2, wherein he retreats to his Осадная пушка in the front of the car, and the sides of the train car open up allowing access to the middle and front portions of the encounter area.

One of the time consuming portions of Phase 2 involves killing adds, looting them and using Артиллерийский снаряд on the Осадная пушка. Each shell does 25% of the cannon's health, so 4 shells alone are required to break Nitrogg out (or 3 shells and another 25% of direct damage to the cannon).

A Охотник can used glyphed Сдерживание to the same effect, but a single hunter may not reach 9 stacks of Взрыв шлака on the Осадная пушка, even with 2 charges of deterrence. Two hunters would definitely be effective in doing so. Even if not quite hitting 9 stacks, a hunter performing this trick will still shorten the length of the fight (likely requiring only 2 Артиллерийский снарядs to break Nitrogg out, rather than 3-4 normally).

It is essential to be quick about moving out of the fire after 9 stacks are reached, as the Взрыв шлака will stack on party members quickly once has expired.

Listed at the end of this post is a WeakAura that can be used to track the stacks of Взрыв шлака, with an alert once 9 stacks are reached.

Make sure the "Unit" dropdown shows "Player, and the "Aura Type" is "Buff"

Thanks to Draemii - Tarren Mill EU, Nexxer - Nordrassil EU, Gothunter - Stormrage US, and Babyjace - Kil'jaeden US for some of the information in this post, including the Взрыв шлака WeakAura above, which is slightly modified from its original form.